Was it a metered taxi? That doesn't sound right. Even if he had to head the other direction for a few blocks (due to one ways) and crossed the river, and then went past the Sheraton and back (familiar with that route), it shouldn't have been more than CL$2.500. It sounds like your driver went to the Buenos Aires School of Taxi Fare Management

Seriously, I'd have called him to the mat on that fare and paid him half.